ENDOGENS AND EXOGENS. 



37 



of the trunk of a palm would reveal a simi- 

 lar structure. Contrast with these stems a 

 cress-section of an oak, as shown in Fig. 44. 

 In this section there are conspicuous layers 

 or rings of wood ; the internal threads are 

 not to be seen. The corn-stalk and the 

 trunk of the palm increase in diameter by 

 the addition in the interior of new threads 

 which stretch out the surface of the stalk. 

 These plants are inside growers or endogens. 



Fig. 43. 



Fig. 44. 



The trunk of the oak increases in diame- 

 ter by the addition of new wood in layers 

 near its surface. It is, therefore, an outside 

 grower, or an exogen. In the Northern 

 United States the endogens are all herbs, 

 with the single exception of the straggling 

 green-brier or smilax. In warmer climates 

 the endogens are represented largely by 

 palms and similar plants. It is evident 



